In fact the resistivity of wood varies with wood species, moisture content of the wood and with temperature. Wood moisture meters are used to approximate the actual moisture content of the wood and the pin type are basically specialized ohm meters. They measure the resistance between two pins inserted into the wood fibers. The resistance value is then converted to moisture content and displayed. The meters have different conversion factors depending on the wood species and temperature.

In fact the resistivity of wood varies with wood species, moisture content of the wood and with temperature. Wood moisture meters are used to approximate the actual moisture content of the wood and the pin type are basically specialized ohm meters. They measure the resistance between two pins inserted into the wood fibers. The resistance value is then converted to moisture content and displayed. The meters have different conversion factors depending on the wood species and temperature.

The same way you measure the resistivity of a wire or anything else. Measure the resistance, determine the cross section area and the length. Then use the formula to calculate resistivity. But that calculated resistivity only holds for the specific species, temperature and moisture content at the time of measurement since wood is hygroscopic and its resistivity varies widely.

"Pin or resistance meters read the moisture content of the wettest area that contacts the uninsulated part of the pin/electrode. Effective measurement range of resistance or pin meters is from 7 to 30% MC. These meters are specialized ohm meters, and below 7% MC, the electrical resistance is too high for reliable readings. For example, at 7% MC the resistance for red oak is about 15,000 megohms, and for hard maple 72,000 megohms. Above 30% MC, the electrical resistance has too much variability and too little change for reliable readings. At 30% MC the resistance for red oak is about 0.50 megohms and for hard maple it is about 0.60 megohms." (R. S. Boone and E. M. Wengert)

"Pin or resistance meters read the moisture content of the wettest area that contacts the uninsulated part of the pin/electrode. Effective measurement range of resistance or pin meters is from 7 to 30% MC. These meters are specialized ohm meters, and below 7% MC, the electrical resistance is too high for reliable readings. For example, at 7% MC the resistance for red oak is about 15,000 megohms, and for hard maple 72,000 megohms. Above 30% MC, the electrical resistance has too much variability and too little change for reliable readings. At 30% MC the resistance for red oak is about 0.50 megohms and for hard maple it is about 0.60 megohms." (R. S. Boone and E. M. Wengert)

My guess is you are not going to be able to do this effectively for wood. First, wood is not a homogenous substance. There is considerable variance in any one piece of wood. Resistivity will vary greatly depending on where and how it is measured. Second, each different species of wood will have a very different bulk resistivity (given all other things being equal). Third, the mineral and salt content of were the wood was grown will greatly affect its resistivity. Fourth, humidity and moisture level will also have a huge effect. Fifth, but not final, the age and general density of the wood sample will have a large effect. There are just too many variables.

When you measure the cross sectional area, you have to look at which way your passing your test current through. Obviously it does not matter if you have something like a cube but for any asymmetrical shape this would apply.

have you learned what radius is in school yet? it's the distance from the center of a circle to the outside. for example, if you have a circle 4cm across, the radius is 2cm. so the area of that circle is 2cm*2cm*3.14159 = 12.57cm²

Bill I think you are running the circumference and the area formulas for circles together into one. area of a circle is r². circumference is 2r.http://www.mathsisfun.com/area.html

have you learned what radius is in school yet? it's the distance from the center of a circle to the outside. for example, if you have a circle 4cm across, the radius is 2cm. so the area of that circle is 2cm*2cm*3.14159 = 12.57cm²

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Ah, I see. That's why every return I had when I am googling are circle because of that.

Um, what if the thing i want to measure its resistance is a square. how? rather, a triangle?